Currently, electric energy-storage devices having large capacity are required in the fields such as midnight-power storage and auxiliary power supplies for power failure. Electric energy-storage devices having large capacity per unit volume, that is, having abilities of supplying power for long duration in spite of the small size thereof, are also required in the field of transportation vehicles characterized by battery-powered driving force such as battery-powered electric vehicles and hybrid electric vehicles and in the field of portable electric terminals such as mobile personal computers, cellular phones and portable audio devices.
Electric double-layer capacitors are expected as an electric energy-storage device, which are composed of an electrode, a separator and an electrolytic solution and store electric power in a boundary surface (electric double-layer) formed between an electrolyte and an electrode due to absorption of the electrolyte dissolved in an the electrolytic solution to the electrode. The capacity of stored energy is defined by the formula ½·C·V2 (wherein C is electrostatic capacity (F) and V is voltage), therefore in order to store more energy, the electrostatic capacity of electric energy-storage devices must be enhanced, especially in order to store more energy in compact volume, enhancement of electrostatic capacity per unit volume is required.
Activated carbons are commonly used for the electrode of electric double-layer capacitors, specifically included is an activated carbon having micropores (equal to or less than 20 Å of pore diameter) as major pores thereof which is obtained by carbonizing and activating palm shell and the like. U.S. Pat. No. 4,873,218 discloses in its table 1 an activated carbon produced from the organic aerogel obtained by polymerizing under the conditions that molar ratio (R/C) of resorcinol (R) to basic catalyst (C) is 200 to 410 and weight ratio (R/W) of resorcinol (R) to aqueous solvent (W) is 0.027 to 0.067, Kohyo (Published Japanese translations of PCT international publication for patent applications) No. 2002-511899 discloses in its claim 1 and Examples 1 to 4 an activated carbon produced from the organic aerogel obtained by polymerizing under the conditions that (R/C) is 2000 or more and (R/W) is 0.39 to 0.56, however, development of electric double-layer capacitors applying a novel activated carbon capable of more enhancing electrostatic capacity per unit volume is desired.
Recently, an activated carbon having meso-pores as major pores thereof is disclosed which is produced by polymerizing resorcinol with formaldehyde in the presence of a basic catalyst and an aqueous solvent to obtain an organic aerogel uniformly having meso-pores (equal to or more than 20 Å of pore diameter), washing the organic aerogel with an organic solvent, replacing the aqueous solvent to the organic solvent, drying and then carbonizing, and can be used for the electrode of electric double-layer capacitor having large electrostatic capacity per unit weight (refer to Kokai (Publication of unexamined patent applications) No. H9-328308 ([0017]), and Kohyo No. 2002-511899 (page 35)).
In the Kokai No. H9-328308 ([0017]), tetraethylammonium salt is used as an electrolyte, and in the Kohyo No. 2002-511899 (page 35), potassium hydroxide is used as an electrolyte, however, application of aqueous electrolytic solution such as potassium hydroxide for the electrolyte causes problems such as poor voltage endurance (voltage) and less storable energy capacity.
The present inventors studied the electric double-layer capacitor disclosed in the Kokai No. H9-328308 ([0017]), and found that this capacitor is disadvantageous to give sufficient electrostatic capacity per unit volume.
The present inventors have studied electric double-layer capacitors having few problems mentioned above, and found that electric double-layer capacitors containing a kind of electrolyte or a kind of activated carbon have enhanced electrostatic capacity per unit volume.